your account might have been temporary disconnection and just reconnected last May 12 thus you were charged of prorated amount from May 12-26. the May 27 - Jun 26 is normal as you will be charged in advance and will this will be recomputed once your account is temporarily disconnected or terminated. yan yong 1 month advance payment mo.. nagamit na siguro nong na temporary disconnect ka kaya need mo ulit ang 1 month advance.. you may check your previous bills to confirm if na gamit mo na.